This is an attempt to avoid the crashes in #341.

It won't fix the underlying issue but I hope it will avoid it in most cases.
The various crash reports indicate the underlying download operation is being
freed before the async block in dataReceived is being executed.

This fix change tries to avoid every calling the async block.

@@ -160,10 +160,7 @@
             self.progressBlock(0, expected);
         }
 
-        dispatch_async(self.queue, ^
-        {
         self.imageData = [NSMutableData.alloc initWithCapacity:expected];
-        });
     }
     else
     {
@@ -182,12 +179,12 @@
 
 - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
 {
-    dispatch_async(self.queue, ^
-    {
     [self.imageData appendData:data];
 
     if ((self.options & SDWebImageDownloaderProgressiveDownload) && self.expectedSize > 0 && self.completedBlock)
     {
+        dispatch_async(self.queue, ^
+        {
             // The following code is from http://www.cocoaintheshell.com/2011/05/progressive-images-download-imageio/
             // Thanks to the author @Nyx0uf
 
@@ -254,7 +251,8 @@
             }
 
             CFRelease(imageSource);
-        }
+        });
+        
         NSUInteger received = self.imageData.length;
         dispatch_async(dispatch_get_main_queue(), ^
         {
@@ -263,7 +261,7 @@
                 self.progressBlock(received, self.expectedSize);
             }
         });
-    });
+    }
 }
 
 - (void)connectionDidFinishLoading:(NSURLConnection *)aConnection